The X-Files contained countless mysteries during its 11-season run, but perhaps none was as compelling as the unusual disappearance of Fox Mulder's younger sister, Samantha. Believing her to be abducted by aliens, Mulder establishes his sister's disappearance as the motive for taking on the X-Files, gradually developing an elongated obsession with finding her. But it's not until Season 7 of The X-Files that the truth is finally revealed about what happened to Samantha Mulder.
Source：https://www.cbr.com/x-files-samantha-mulder-what-happened/
